This invention relates to amorphous metal alloys. More specifically, this invention relates to amorphous iron-rich and nickel-rich chromium alloys which are corrosion-resistant, are stable to relatively high temperatures and which can be applied as coatings for the protection of less corrosion-resistant materials.
Highly corrosive environments require the use of materials which are able to withstand corrosive attack from these environments for extended periods of time. For example, blades and other components in turbines used to generate electrical power from steam recovered from geothermal sources must be able to function in an environment containing high concentrations of sulfur dioxide, chloride ions and other highly corrosive materials.
Corrosion-resistant coatings of amorphous iron-chromium and iron-chromium-nickel based alloys are presently available for the protection of substrates which are subject to attack by their environment. Most of these alloys are stabilized in the amorphous state by one or more of the metalloid elements such as B, C, Si and P. Amorphous or glassy alloys such as these, are very resistant to corrosive attack in a neutral or acid environment at temperatures below about 200.degree. C. At temperatures from 200.degree. to 400.degree. C. the alloy is stable, although it becomes less resistant to corrosive attack. However, annealing most of the alloys at temperatures above 400.degree. C. completely crystallizes the alloy. Once crystallized, the alloy loses its chemical inertness and is subject to corrosive attack just as any normal alloy would be.
In the Journal of Non-Crystalline solids 29 (1978), pages 61-65, the addition of Mo and W to a series of amorphous iron-chromium and iron-chromium-nickel-based stainless steel alloys stabilized with P and C is described. The article discloses that, although the corrosive rate was decreased due to the presence of the Mo and W in the alloy, it was the chromium which provided the most dramatic increase in corrosion resistance. Furthermore, the alloys were stabilized in the amorphous state by P and C, and were subject to crystallization when heated to the higher temperatures.
Thin films of amorphous 304 stainless steel (74% Fe, 18% Cr, 10% Ni, plus small amounts of P, Mn, S and Si) have also been reported in the Journal of Materials Science, 13 (1978) Letters. The films were sputter-deposited on biased substrates cooled to -196.degree. C. or maintained at near room temperature. The films were said to remain amorphous at temperatures up to 800.degree. to 900.degree. C. The films, however, were extremely thin, i.e. about 200 nm thick, to prevent crystallization and therefore are not suitable for substrate protection.